This invention relates to transducers for flow monitoring equipment and more particularly relates to a transducer employing mode conversion for a flow monitoring application.
Transducer structures for flow monitoring equipment are known and are shown, for example, in U.S. Pat. Nos. 3,869,915 and 3,987,674, each of which is in the name of Joseph Baumoel. Transducer structures for such flowmeters are also shown in copending application Ser. No. 146,530, filed May 5, 1980 in the name of Joseph Baumoel, now U.S. Pat. No. 4,373,401.
The above patents and application disclose a transducer system in which ultrasonic energy is injected into a conduit over a relatively long axial distance of the conduit so that the receiving transducer can receive a usable signal without having to be at an exact axial location relative to the transmitting transducer. This is done by causing a shear mode sonic wave to propagate along the pipe wall in response to a longitudinal mode sonic wave in the transducer housing. Thus, the longitudinal wave from the transducer housing is applied at an agle to the pipe axis so that the phase velocity of the longitudinal transducer housing wave arriving at the pipe surface is equal to the pipe shear wave velocity. In particular, the sine of the angle between the transducer crystal and the pipe surface is made equal to the ratio of the longitudinal mode sonic velocity of the housing to the shear mode velocity of the conduit. This produces good coupling between the transducer housing and the pipe housing and introduces considerable energy along the axis of the pipe to produce what is known as a large "footprint".
In order to carry out this concept, it is necessary that the longitudinal mode sonic velocity of the transducer housing be less than the shear mode sonic velocity of the pipe or conduit. This is required since the phase velocity of the longitudinal wave must be increased to the shear wave velocity in the pipe wall. Because of this requirement, when the conduit is made of metal such as steel, which has a high shear wave velocity of approximately 126.times.10.sup.3 inches/second, the transducer body had to be of a non-metallic material having a longitudinal sonic velocity less than that of the metal pipe.
This has prevented the use of metal transducer housings in the past and required the use of materials such as plastics for the transducer housing. Plastics, however, are generally limited in their operating temperature range so that the system described above was limited to relatively low temperature applications or required the use of esoteric and expensive plastics such as polyimid amides. Thus, while it was desirable to employ metallic transducer housings which have very stable temperature characteristics, metal transducer housings have not been used, prior to this invention, because their longitudinal mode sonic velocity is higher than the shear mode velocity of a metal conduit.
A principal object of this invention is to provide a novel arrangement which permits metals such as brass, zinc, copper and the like to be used for the transducer body of an ultrasonic measuring system in which the transducers are clamped directly to the conduits or pipes which carry fluid, the characteristics of which are to be measured or determined, which conduits may be of ferrous or other metal material and which may be at very high temperature.